The present invention relates to a semiconductor device and a method of forming the same, and more particularly to an improved contact plug having an ohmic contact with a silicon substrate and a method of forming the same.
As the advanced large scale integrated circuits have been developed to further increase the density of integratio, the requirement for size down of a contact plug has been on the increase, wherein the contact plug is provided in a contact hole formed in an inter-layer insulator over a silicon substrate so that an interconnection extending over the inter-layer insulator is electrically connected through the contact plug in the contact hole to a semiconductor device formed in an upper region of the silicon substrate. A thickness of the inter-layer insulator is not reduced despite scaling down of the semiconductor device and despite the increase in density of the integratio of the large scale integrated circuits. This means that the depth of the contact hole or the height of the contact plug is also not reduced. Notwithstanding, a diameter or a horizontal size of the contact hole or the contact plug is reduced upon scaling down of the semiconductor device and also upon the increase in density of the integratio of the large scale integrated circuits. An aspect ratio of the contact hole is defined to be a ratio of a depth of the contact hole to a diameter of the contact hole, wherein the depth of the contact hole corresponds to a thickness of the inter-layer insulator in which the contact hole is formed. The recent further increased requirement for scaling down of the semiconductor device and increase in the density of integratio of the large scale integrated circuit results in an increase in the aspect ratio of the contact hole. This increase in the aspect ratio of the contact hole makes it difficult to reduce a resistance of the contact plug in the contact hole. In general, in order to obtain a possible ohmic contact of the contact plug with a silicon substrate of both type, namely n-type and p-type, a thin titanium film is deposited on a bottom and a side wall of the contact hole prior to a further deposition of a thin titanium nitride film as a barrier metal on the thin titanium film to form laminations of the titanium film and the titanium nitride film, before a tungsten film is deposited by a chemical vapor deposition on the titanium nitride film as the barrier metal so as to deposit the tungsten film within the contact hole, whereby a contact plug is then formed in the contact plug, wherein the contact plug comprises the tungsten film and the laminations of the titanium nitride film and the titanium film. The tungsten film deposited by the chemical vapor deposition has a good contact coverage. It is required that each of the titanium film and the titanium nitride film has a thickness of not less than 10 nanometers on the bottom of the contact hole. If the thickness of the titanium film on the bottom of the contact hole is less than 10 nanometers, then a contact resistance of the plug with the silicon substrate is high. If the thickness of the titanium nitride film on the bottom of the contact hole is less than 10 nanometer, this extremely thin titanium nitride film is incapable of exhibiting a barrier property to tungsten hexa-fluoride as a source gas for carrying out the chemical vapor deposition to deposit the tungsten film, thereby allowing tungsten hexa-fluoride to be reacted with titanium nitride of the titanium nitride film and titanium of the titanium film, resulting in an increase in the contact resistance of the contact plug and a possible breaking of the p-n junction.
In the advanced semiconductor devices such as 64 Mb-DRAM, the aspect ratio of the contact hole exceeds 4. In this case, it is difficult to apply or use the normal sputtering method to deposit the titanium film or the titanium nitride film, both of which have a thickness of not less than 10 nanometers. In order to obtain a good contact coverage, it is required that sputtered particles fall in a direction just or almost vertical to a surface of the substrate or in a parallel direction to the vertical side wall of the contact hole. In order to respond to this requirements, a collimated sputtering method and a long distance sputtering method are available.
The collimated sputtering method is carried out by using a collimator plate between a target to be sputtered and a substrate on which sputtered particles are deposited, wherein the collimator plate has many openings through which sputtered particles pass. Only when the sputtered particles fall in the direction just or almost vertical to the surface of the substrate, then the sputtered particles are allowed to pass through the collimator plate. Otherwise, the sputtered particles are captured by the collimator plate. For those reasons, only the sputtered particles, which fall in the direction just or almost vertical to the surface of the substrate, will be deposited on the surface of the substrate or on the bottom of the contact hole having the high aspect ratio. This technique is, for example, disclosed in Japanese laid-open patent publication No. 5-299375.
The long distance sputtering method is carried out by setting such a large distance between a target and a substrate that only sputtered particles, which fall in the direction just or almost vertical to the surface of the substrate, can reach on the surface of the substrate, whilst other sputtered particles having a lateral velocity component will be captured by shielding plates.
It was also known in the art to which the present invention pertains that after the titanium film and the titanium nitride film have been deposited, then a heat treatment is carried out in a nitrogen atmosphere to cause a silicidation reaction of titanium with silicon on an interface between the titanium film and the silicon substrate, so that a titanium silicide film is formed on the interface of the silicon substrate with the titanium film, whereby a contact resistance between the plug and the silicon substrate is reduced. This heat treatment can concurrently cause a nitration reaction of unreacted titanium with nitrogen in the titanium nitride film, thereby increasing the barrier capability of the titanium nitride film as a barrier metal. This heat treatment may be carried out by using a furnace as disclosed in the above mentioned Japanese laid-open patent publication No. 5-299275 or by use of a lamp anneal system for rapid anneal in a short time and this lamp anneal system is suitable for a large size substrate.
The following descriptions will be directed to a conventional method of forming the contact plug in the contact hole of the high aspect ratio, wherein the collimated sputtering method is utilized.
Semiconductor devices are formed in a silicon substrate. A boro-phospho-silicate glass film as an inter-layer insulator is deposited by a chemical vapor deposition method on a surface of the silicon substrate. A contact hole is formed in the inter-layer insulator by use of a photo-lithography and a subsequent dry etching process so that the contact hole reaches a surface of the silicon substrate, whereby a part of the surface of the silicon substrate is exposed through the contact hole. The contact hole has a diameter of not larger than 0.35 micrometers.
The collimator sputtering method is carried out to deposit a titanium film on the top surface of the inter-layer insulator and on vertical side walls and a bottom of the contact hole, wherein the titanium film has a thickness in the range of 50-100 nanometers. Subsequently, a titanium nitride film is then deposited by the collimator sputtering method on the titanium film, wherein the titanium nitride film has a thickness in the range of 80-150 nanometers. This collimator sputtering method is carried out by use of a collimator plate which has an aspect ratio of 1.5, wherein the aspect ratio is defined to a ratio of a thickness of the collimator plate to a diameter of openings. The first collimator sputtering process for depositing the above titanium film is carried out by sputtering a titanium target with argon gas under a pressure in the range of 0.5-3.0 mTorr at a substrate temperature in the range of about 150-400xc2x0 C. The second collimator sputtering process for depositing the above titanium nitride film is carried out by sputtering a titanium nitride target with a nitrogen-argon mixture gas or a nitrogen gas under a pressure in the range of 1.5-5.0 mTorr at a substrate temperature in the range of about 150-400xc2x0 C.
Subsequently, a lamp anneal is carried out in a nitrogen atmosphere at a temperature in the range of 650-720xc2x0 C. for 10-60 seconds, whereby a titanium silicidation reaction of titanium in the titanium film with silicon in the silicon substrate is cased to form a titanium silicide film on an interface between the titanium film and the silicon substrate, thereby reducing a contact resistance of the titanium film with the silicon substrate. This lamp anneal further causes a nitratio reaction of unreacted titanium with nitrogen in the titanium nitride film thereby increasing a barrier property of the titanium nitride film.
A low pressure chemical vapor deposition method is carried out by reducing tungsten hexafluoride with hydrogen to deposit a tungsten film on the titanium nitride film so that the tungsten film has a thickness in the range of 400-600 nanometers. The surface of the CVD-deposited tungsten film is then subjected to an etch-back process with use of sulfur hexafluoride to etch the CVD-deposited tungsten film except within the contact hole, whereby a tungsten contact plug is formed in the contact hole.
An aluminum alloy film is deposited by a sputtering method on the titanium nitride film and over the inter-layer insulator and on the top of the contact plug in the contact hole. Further, a titanium nitride film is deposited by a subsequent sputtering method on the aluminum alloy film to form laminations of the aluminum alloy film and the titanium nitride film. The laminations of the aluminum alloy film and the titanium nitride film are then patterned by a photo-lithography and a subsequent dry etching process thereby to form an interconnection over the inter-layer insulator, wherein the interconnection is in contact with the top of the contact plug in the contact hole so that the interconnection is electrically connected through the contact plug in the contact hole to the semiconductor device in the surface of the silicon substrate.
The titanium nitride film as the barrier metal deposited by the collimated sputtering method has a large compressive stress of, for example, not less than 1xc3x971010dyne/cm2 as compared to the titanium nitride film deposited by the normal sputtering method. This has already reported in 1992, VLSI Multilevel Interconnection Conference, pp. 310-313 xe2x80x9cCharacterization Of Improved TiN films By Controlled Divergence Sputteringxe2x80x9d.
If the barrier metal film has a large compressive stress, then a p-n junction property or characterization is deteriorated by a heat treatment, for which reason it is preferable that the compressive stress of the barrier metal film is suppressed at not higher than 5xc3x97109dyne/cm2. This is, for example, disclosed in Japanese laid-open patent publication No. 63-111665.
The large compressive stress of the barrier metal may cause other problems with appearance of peeling or cracking the titanium nitride film during the later heat treatment. This is, for example, disclosed in Japanese laid-open patent publication No. 2-133964. Particularly when the rapid heat treatment is carried out, then the cracking is likely to appear. The reason why the cracking appears in the titanium nitride film as the barrier metal film is that carrying out a heat treatment to the titanium nitride film as the barrier metal film having such the large compressive stress causes an increase in density of the titanium nitride film to change the large compressive stress into a large tensile stress which may be capable of cracking the titanium nitride film. Particularly rapid thermal anneal such as the lamp anneal to the titanium nitride film as the barrier metal film having such the large compressive stress causes a rapid increase in density of the titanium nitride film to rapidly change the large compressive stress into a large tensile stress, whereby a rapid plastic deformation appears to crack the titanium nitride film.
FIG. 1 is a fragmentary and enlarged cross sectional elevation view illustrative of a bottom portion of a contact hole in an inter-layer insulator and over a titanium nitride film in an upper region of a silicon substrate after a heat treatment has been carried out to cause a silicidation reaction for forming the titanium silicide film. A boro-phospho-silicate glass film 2 as an inter-layer insulator is formed on a silicon substrate 1. A contact hole having a high aspect ratio is formed in the inter-layer insulator 2 by use of a photo-lithography and a subsequent dry etching process so that the contact hole reaches onto a surface of the silicon substrate. A titanium film 3 is deposited on the bottom and the side walls of the contact hole and on a surface of the inter-layer insulator by a sputtering method, for example, a collimator sputtering method for obtaining a good step overage of the titanium film 3. Subsequently, a titanium nitride film 4 is then deposited on the titanium film 3 by a further sputtering method, for example, a collimator sputtering method for obtaining a good step overage of the titanium film 3. In order to reduce a contact resistance of the titanium film 3 with the silicon substrate 1, a heat treatment, for example, a rapid thermal anneal such as a lamp anneal is carried out in an inert gas atmosphere such as a nitrogen gas atmosphere so as to cause a silicidation reaction of titanium with silicon on an interface between the titanium film 3 and the silicon substrate 1, so that a titanium silicide film 5 is formed on the interface of the silicon substrate 1 with the titanium film 3, whereby a contact resistance between the titanium film 3 and the silicon substrate 1 is reduced. Concurrently, a nitration reaction of unreacted titanium with nitrogen in the titanium nitride film 4 is caused, thereby increasing the barrier capability of the titanium nitride film 4 as a barrier metal film. The heat treatment to the titanium nitride film 4 as the barrier metal film causes an increase in density of the titanium nitride film 4, thereby to change the compressive stress of the titanium nitride film 4 into a tensile stress which forms a crack at a corner of the titanium nitride film 4. The crack is likely to appear at the corners of the titanium nitride film 4. Once the cracking appears in the titanium nitride film 4, then tungsten hexafluoride gas used for a subsequent chemical vapor deposition to deposit a tungsten film to bury the contact hole with the tungsten film enters into the crack in the titanium nitride film 4 so that tungsten hexafluoride may be reacted with titanium in the titanium film 3 or silicon in the silicon substrate 1, whereby a contact resistance at the bottom of the contact hole is increased and also a p-n junction may be broken.
As described above, a likelihood of appearance of the crack in the titanium nitride film 4 largely depends upon a magnitude of a compressive stress of the titanium nitride film 4 before the heat treatment. In Japanese laid-open patent publication No. 2-133964, it is disclosed to add carbon into the titanium nitride film in order to reduce the compressive stress of the titanium nitride film. It is, actually, however, difficult to practice this method because of both a difficulty in controllability of addition of carbon into the titanium nitride film and also etching remainders are likely to appear.
The above problems with the titanium nitride film are also raised not only when the collimator sputtering method is applied to deposit the titanium nitride film but also when the other sputtering methods including the long distance sputtering method and the normal sputtering method are applied to the deposition of the titanium nitride film. Namely, the compressive stress of the titanium nitride film is changed by the heat treatment into the tensile stress which causes a crack in the titanium nitride film. Particularly, when the temperature rising rate of the heat treatment is not less than 100xc2x0 C./min, then the crack is likely to appear in the titanium nitride film. The titanium nitride film deposited by the long distance sputtering method has a large compressive stress of, for example, not less than 1xc3x971010dyne/cm2 as compared to the titanium nitride film deposited by the normal sputtering method. This large compressive stress of the titanium nitride film is changed by the heat treatment into a large tensile stress which has a larger capability of causing a crack in the titanium nitride film.
In the above circumstances, it had been required to develop a novel sputtering method of depositing laminations of a titanium film and a titanium nitride film on a bottom of a contact hole having a high aspect ratio so that the titanium nitride film has a reduced compressive stress for allowing the deposited titanium nitride film to be free from any cracking during a subsequent rapid heat treatment to form a titanium silicide film on an interface between the titanium nitride film and the silicon substrate surface, thereby decreasing a contact resistance at a bottom of the contact hole.
Accordingly, it is an object of the present invention to provide a novel sputtering method free from the above problems.
It is a further object of the present invention to provide a novel sputtering method of depositing laminations of a titanium film and a titanium nitride film on a bottom of a contact hole so that the titanium nitride film has a reduced compressive stress for allowing the deposited titanium nitride film to be free from any cracking during a subsequent heat treatment to form a titanium silicide film on an interface between the titanium nitride film and the silicon substrate surface, thereby decreasing a contact resistance at a bottom of the contact hole.
It is a still further object of the present invention to provide a novel collimator sputtering method of depositing laminations of a titanium film and a titanium nitride film with a good contact coverage on a bottom of a contact hole having a high aspect ratio so that the titanium nitride film has a reduced compressive stress for allowing the deposited titanium nitride film to be free from any cracking during a subsequent rapid thermal anneal such as a lamp anneal to form a titanium silicide film on an interface between the titanium nitride film and the silicon substrate surface, thereby decreasing a contact resistance at a bottom of the high aspect ratio contact hole.
It is yet a further object of the present invention to provide a novel long distance sputtering method of depositing laminations of a titanium film and a titanium nitride film with a good contact coverage on a bottom of a contact hole having a high aspect ratio so that the titanium nitride film has a reduced compressive stress for allowing the deposited titanium nitride film to be free from any cracking during a subsequent rapid thermal anneal such as a lamp anneal to form a titanium silicide film on an interface between the titanium nitride film and the silicon substrate surface, thereby decreasing a contact resistance at a bottom of the high aspect ratio contact hole.
It is a further more object of the present invention to provide a novel method of forming a contact plug in a contact hole free from the above problems.
It is still further object of the present invention to provide a novel method of forming a contact plug in a contact hole by a novel sputtering method of depositing laminations of a titanium film and a titanium nitride film on a bottom of a contact hole so that the titanium nitride film has a reduced compressive stress for allowing the deposited titanium nitride film to be free from any cracking during a subsequent heat treatment to form a titanium silicide film on an interface between the titanium nitride film and the silicon substrate surface, thereby decreasing a contact resistance at a bottom of the contact hole.
It is moreover object of the present invention to provide a novel method of forming a contact plug in a contact hole by a novel collimator sputtering method of depositing laminations of a titanium film and a titanium nitride film with a good contact coverage on a bottom of a contact hole having a high aspect ratio so that the titanium nitride film has a reduced compressive stress for allowing the deposited titanium nitride film to be free from any cracking during a subsequent rapid thermal anneal such as a lamp anneal to form a titanium silicide film on an interface between the titanium nitride film and the silicon substrate surface, thereby decreasing a contact resistance at a bottom of the high aspect ratio contact hole.
It is another object of the present invention to provide a novel method of forming a contact plug in a contact hole by a novel long distance sputtering method of depositing laminations of a titanium film and a titanium nitride film with a good contact coverage on a bottom of a contact hole having a high aspect ratio so that the titanium nitride film has a reduced compressive stress for allowing the deposited titanium nitride film to be free from any cracking during a subsequent rapid thermal anneal such as a lamp anneal to form a titanium silicide film on an interface between the titanium nitride film and the silicon substrate surface, thereby decreasing a contact resistance at a bottom of the high aspect ratio contact hole.
It is still another object of the present invention to provide a novel titanium nitride film deposited by a novel sputtering method on a titanium film so that the titanium nitride film has a reduced compressive stress free from the above problems.
It is yet another object of the present invention to provide a novel titanium nitride film deposited by a novel sputtering method on a titanium film on a silicon substrate so that the titanium nitride film has a reduced compressive stress for allowing the deposited titanium nitride film to be free from any cracking during a subsequent heat treatment to form a titanium silicide film on an interface between the titanium nitride film and the silicon substrate surface, thereby decreasing a contact resistance at a bottom of the contact hole.
It is further another object of the present invention to provide a novel titanium nitride film deposited by a novel collimator sputtering method on a titanium film on a silicon substrate so that the titanium nitride film has a reduced compressive stress for allowing the deposited titanium nitride film to be free from any cracking during a subsequent rapid thermal anneal such as a lamp anneal to form a titanium silicide film on an interface between the titanium nitride film and the silicon substrate surface, thereby decreasing a contact resistance at a bottom of the high aspect ratio contact hole.
It is an additional object of the present invention to provide a novel titanium nitride film deposited by a novel long distance sputtering method on a titanium film on a silicon substrate so that the titanium nitride film has a reduced compressive stress for allowing the deposited titanium nitride film to be free from any cracking during a subsequent rapid thermal anneal such as a lamp anneal to form a titanium silicide film on an interface between the titanium nitride film and the silicon substrate surface, thereby decreasing a contact resistance at a bottom of the high aspect ratio contact hole.
In a first aspect, the invention provides a multilayer structure on a bottom of a contact hole, the multilayer structure comprising a titanium film on a silicon region and a titanium nitride film on the titanium film, wherein the titanium nitride film has a compressive stress of not higher than 5xc3x97109 dyne/cm2 so that the titanium film has such a high stability as preventing any crack upon changing the compressive stress to a tensile stress by a heat treatment.
In a second aspect, the invention provides a contact plug structure in a contact hole in an inter-layer insulator on a silicon substrate. The contact plug structure comprises : a titanium film on a bottom and a side wall of the contact hole; a titanium nitride film on the titanium film; a metal plug on the titanium nitride film so that the metal plug buries the contact hole ; and a titanium silicide layer in a surface region of the silicon substrate and in contact with a bottom of the contact hole, wherein the titanium nitride film free of any crack and has a tensile stress of not higher than 3xc3x97109 dyne/cm2.
In a third aspect, the invention provides a sputtering method of depositing a titanium nitride film on a titanium film in contact with a silicon at a bottom of a contact hole, wherein the sputtering method is carried out at a temperature of the silicon region of not less than 450xc2x0 C., so that the titanium nitride film has a compressive stress of not higher than 5xc3x97109 dyne/cm2 whereby the titanium film has such a high stability as preventing any crack upon changing the compressive stress to a tensile stress by a heat treatment.
In a fourth aspect, the invention provides a method of forming a multilayer structure on a bottom of a contact hole. The method comprises the steps of: carrying out a first sputtering method to deposit a titanium film on the bottom of the contact hole so that the titanium film is in contact with a silicon region ; carrying out a sputtering method to deposit a titanium nitride film on the titanium film; carrying out a heat treatment for causing a silicidation reaction of titanium with silicon to form a titanium silicide film on an interface between the titanium film and the silicon region, wherein the sputtering method is carried out at a temperature of the silicon region of not less than 450xc2x0 C., so that the titanium nitride film has a compressive stress of not higher than 5xc3x97109 dyne/cm2, so that the titanium nitride film is free of any crack upon changing the compressive stress to a tensile stress by the heat treatment.
In a fifth aspect, the invention provides a method of forming a contact plug in a contact hole formed in an inter-layer insulator on a silicon substrate. The method comprises the steps of: carrying out a first sputtering method to deposit a titanium film on the bottom of the contact hole so that the titanium film is in contact with a silicon region; carrying out a sputtering method to deposit a titanium nitride film on the titanium film; carrying out a heat treatment for causing a silicidation reaction of titanium with silicon to form a titanium silicide film on an interface between the titanium film and the silicon region; and forming a metal plug on the titanium nitride film to bury the contact hole with the metal plug, thereby forming the contact plug in the contact hole, wherein the sputtering method is carried out at a temperature of the silicon region of not less than 450xc2x0 C., so that the titanium nitride film has a compressive stress of not higher than 5xc3x97109 dyne/cm2, so that the titanium nitride film is free of any crack upon changing the compressive stress to a tensile stress by the heat treatment.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.